In recent years, flame retardancy of resin materials has become more important because the use of materials is expanding to various applications and to the uses in more densified conditions. Patent Documents 1 discloses the flame-retardant polyester copolymer comprising bifunctional alkylphosphine oxide as a copolymerization component. Patent Documents 2 also discloses the phosphorus-containing polycarbonate characterized by comprising a certain structural unit comprising phosphorus. Since the terminal group of this phosphorus-containing polycarbonate is phenolic hydroxyl group, its usage as a reactive flame-retardant agent has been limited due to low reactivity.    Patent document 1: JP-A-H8-157,584    Patent document 2: JP-A-2003-206,350